Method of electrowinning manganese



Patented Apr. 20, 1948 MANGANESE Herbert It. Hanley, Rolla, Mo., andJames Jacobs, Boulder City, Nev.,

No Drawing. Application August 4,1942;- Serial No. 453,542 It V 2Claims. (o1.'204--105) (Granted under the act of March 3, 1883, as

This invention described herein may be manufactured and used by or forthe Government for governmental purposes, without the payment to us ofany royalty thereon.

Our invention relates to improvements in the art of winning manganesefrom aqueous solutions, and more particularly to an improved anode foruse in the electrodeposition of manganese, the use of which is a markedcontribution to the electrolytic managnese process.

The anode problem in the electrolytic manganese process is of vitalimportance. In a process where metal is plated from solutions usinginsoluble anodes, the choice of the material to be used in the anodes isvery limited. There are .two main anode reactions which mightoccurjwhich would be detrimental to the process.

First, the anode must be insoluble, and second, the reactions occurringat the anode must not interfere with the process. Usually, though notnecessarily, the anode product during electrolysis are oxidationcompounds formed by the strong oxidizing atmosphere of probably nascent,oxygen.

The first consideration, namely, the insolubility of the anodes, is verywell met by the use of lead anodes in sulphate solutions. However, leaddoes not meet the second requirement when used in the electrolyticmanganese process. When manganese sulphate-ammonium sulphate solutionsare electrolyzed using lead anodes, a considerable amount of themanganese in solution is rendered insoluble, perhaps as the oxides, themain one apparently being MnOz.

The following figures are given to show how serious this precipitationof manganese is to the electrolytic manganese process.

The Bureau of Mines reported in their electrolytic manganese work thatfor every three pounds of manganese produced as cathode metal there wasone pound of manganese precipitated at the anodes. This amount ofmanganese precipitated constitutes a very serious problem, not only inkeeping the electrolytic cells in proper operating condition, butfinally in adding considerably to the cost of making cathode c1ectrolytic manganese.

The following figures show how this is true:

Taking 2000 pounds of cathode metal, there would be 667 pounds ofmanganese precipitated at the anodes as insoluble manganese compound;

The precipitate assays approximately 40% Mn, so there wouldbe 1334pounds of dry manganese compounds suspended in the cell :anolyte. Itwould be necessary to filter these compounds from the anolyte, and asthe wet filter cake assays t amended April :30, 1928; 370 G. 757) leastmoisture, there would be roughly 2600 pounds of the wet cake. In orderto wash the ammonium s'ulphatebearing anolyte from the wet cake at leastone ton of waterwould be necessary. This water wash would be a problem,and it is very possible that evaporation of it would be necessary inorder to keep down plant solution' volumes. V

In order to recover the manganese in the wet filter cake a reductionroast would be necessary to convert the high manganesecompounds, such asM17102, to the acid soluble oxide MnO.

The manganese oxides on precipitating in the electrolytic cells alsooiier a serious problem in the fouling of the diaphragm coverings. Also,periodically, the cells would-have to be shut down and cleanedof themanganeseoxides which build up in the cell bottoms.

The above explanation makes clear the desirability oiobtaining an anodewhich will prevent the precipitation or the manganese compounds. Thiskind ofanode would definitely cut down on the cost of producingelectrolytic manganese.

There is one other feature in the electrolytic manganese process,asitnow stands, which is very undesirable and which can be overcome byuse of an anode which would not allow precipitation of the oxides ofmanganese."

As the process is now operated the anodes and cathodes are separated bycloth diaphragms. The reason for this'is that manganese cannot be platedcommercially from highly acid solutions. In the electrolytic cellarrangement the cathodes are placed inside the diaphragms and the anodesoutsi de the diaphragms. As 27 cathodes are usually used in eachelectrolytic cell, and as about 12 cells are necessary to produce oneton of cathode manganese per day, this means 324 cathodes. Each'of thesecathodes has its own independent feed line, and as the solution insidethe diaphragm holds onlyone-half cubic foot of solution it is veryevidentthat considerable supervision must be maintained in order toproperly feed the cells. I Now, withan anode whichdoes not precipitatethe manganese" oxides, the cathodes could be placed outside thediaphragms and the anodes inside the diaphragms. This would cut the feedlines'to the'cells from 2'7 to oneand at the same time insure a moreuniform solution around the cathodes. The volume of solution per cathodejumpsfrom one-halfa cubic foot to four cubic feet, or eight times asmuch; As the cathode metal is the product from cells which is desired,the above change would make for steadier cathode p ting oonditionsg'andat the same time reduce procedure is expensive and the cloth quicklydeteriorates and ruptures.

Colin G. Fink (Anodes for Electrowinning Manganese, The Electrochemical.$00., Preping 76-5;

September 14, 1939) developed an alloy of leadthe sameconditions, the 1%silver-lead anodes so tin-cobalt which will prevent manganese frombecoming oxidized at the anodes. However, this alloy is very expensiveand difficult to prepare, as

several steps are necessary in its preparation.

The principal object of our invention is the provision of a. relativelyinexpensive anode which will solve the problems discussed. 7 I V Theanode of our invention is easytoprepare, is of comparatively low cost,is insoluble in manganese sulphate-ammonium sulphate solutions, andminimizes precipitation of manganese anodic compound... This anodecomprises a lead-silver alloy and research work'overa long periodhasproven its adaptability to the electrolytic manganese process.

A certain amount oithe manganesecompounds are formed initially on thealloy anode. .Preforming is indicated and more iullydiscussed in theexperimental data appended.

Lead-silver alloysv form-a eutectic mixture at approximately 2% to 2%%silver, 97 72,. to 98% lead (International Critical Tables, vol. 2, p.414; Metals Handbook, Am. Soc. for Metals, 1939 ed'., Cleveland, Ohio,p. 1523). We propose usually to use silver ina concentration about theeutectic or below. Experience in other fields ithe use of Pb-Ag alloyanodes is old, e. g., U. S. 1,851,219; 1,759,493 and 1,587,438) seems toindicate that higher proportions of silver may be undesirable andthe'cost of silyer would appear-to render it inadvisable toexceed about3%,- sofar assilver content is concerned, a proportion. which we havefound experimentally togive very desirableresuits, We; therefore, donotlimit ourselves to the use of very low percentages of silver inthepractice of our invention.

The practice of our invention may be illustrated by referencete-actualtest data given by way of example;

Several anodes were preparedby melting-tgether 99 of lead and 1%of'srlver and. casting the alloy so produced into an open: mold. Theanodes s0 produced analyzed approximately 1% silver, balance lead. Ascast, they were 1%." thick; and were trimmed to have a surfaceareasomewhat less,- thanthe surface area of. the oath odes with whichthey were used. Several anodes so produced werermounted in a cell, andthe anodes preformed by" operating the cell for 24 hours; During.the'flrstiourj hours manganese was precipitated atsucliianodes; and atthe endof this period the" surface was covered-with a blue-black'filmoniwhich a dark; scale had started to form. 'I'hisrscalezincrea'sedduring the prefor'rning' period. At? the; endtofifthe period" the scalewas removecr'learej being exercised not: to rupture the.blue-blaclcfilmrzand theianodes werethen'weighed- The ce1ls,"using7 theanodes: prepared as above, were" operatedusing .asolution containing 140grams perliter ofam-moniuin sulphate and 25 grams per liter of manganeseasthe sulphate and 0.2 to- 0.5-gra-rn=perli-ten of sulphur dioxide-inthe- :Eeed. A- ten gram strip was. taken. and; the an lyte' contained 15grams per liter of manganese and the acid concentration was about 20grams per liter. The cells were operated with a current density at theanodes of either 28 amperes or 58 amperesper square foot.

As the cell was operated there was a very small amount of manganesedioxide formed at the anode, but after the first days operations, theanolyte was very clear. A dark brown scale built up On the anode surfaceas plating continued.

Comparedwith other anodes employed under produced permitted operation atsatisfactory 'curren-t efiiciency, that is, consistently above and itappears that more than adequate current efiiciencies may be maintained.The tests ran were not particularly concerned with currentefificiencies, however, but rather with Mp0s formation and anode losses.The latter were very low, and unquestionably may be reduced ,further byselection of most optimum conditions;

In another series of tests, we produced a number of anodes containing 2/2% silver, balance lead, and treated them in the same manner asdescribed in connection with the '1% silver anodes. These alsoshoweddesirable results from the standpoint of efficiencies and costs,particularly from the standpoint of low MnOz production and low anodelosses.

Other anodes containing other low percentages of silver produce the samedesirable advantages, but in our experimental work we utilize'dprimwrily 1%.and 2 /g% silver' and ran many tests to determine the relativeadvantageous characteristics of thesetwo embodiments.

For the benefit'ofthose skilled in the art, we show below the results ofoperations over a period of approximately two. weeks on certain anodesproduced in accordance with the invention, these anodes havingv beenselected at random from the group produced, and being identified in thethree tables by number. {In the tables, 0. D. stands for current.density in amperes per square foot.

The other symbols are conventional.

TABLE I C'ell data 4 Amp. Wt. Anode Percent. Anode Wt-AHOdG HoursCathode No. Ag G. D. Operated MIL g Scale g.

a 1 26 4725 3135. 7 133. 5 4. 1 28 4725 3125.7 1179.0 25 2a; 1 47252956.7 218.5 2.5 28 4725 2966.7 196.5 1 5a 3363 1958 49.0 1 53 3363 195840.0 2. 5 58 3363 1051. 2 43. 0 2. 5 58 3363 1981. 2 45. 5

TABLE II Analysis: of anode scale Anode-No: fg 3 Pb. M11. Ag Ca TABLEIII Calculatzons from cell data Ratio Pep Per. of Mn cent Per- Per- AgPb Anode m Anode 1n Loss in cent cent Loss Loss No. A C. D. Scale AnodeLoss Loss 'lon Ton g to Mn Wt Ag Pb Mn Mn Dep.

From the results shown in the preceding tables it can be seen that whenthe cells are operated with the higher current density on the anode-s,the amount of scale formed is markedly decreased, thereby lowering thesilver and lead loss and the amount of manganese precipitated at theanode. The average voltage for the 1.0% and 2.5% silver anodes at 28current density was 4.00 volts, and at 58 current density it was 4.26volts. The increased power cost due to the higher operating voltage whenusing the high current density on the anodes would more than becompensated for by the lower anode cost resulting from the small loss oflead and silver.

Very little difierence could be determined between the efiiciency of 1%and 2.5% Ag anodes and, since the initial investment would be smaller ifthe 1% Ag anodes were used, it is believed that on the basis of thesetests, the 1% Ag anode would be the most logical composition to use.Although scale formedduring the preforming in the tests, this would notbe necessary in actual practice. The preforming of the anodes was donemerely to keep the cells clear of precipitated manganese compounds, butit could be dispensed with without damaging the electrolytic circuit.The calcium present in the anode scale was due to gypsum crystals beingprecipitated upon the anode. cuts of the scale is obvious.

The results show definite trends and represent the maximum losses thatwould be encountered with silver-lead anodes. It is believed that, asthe time of operation is increased after the darkbrown scale is formed,the silver and lead losses would be greatly decreased. Based solely uponthe results shown, however, the anodes of the present invention aredefinitely superior to other anodes, such as the lead anodes of theprior art, and in some respects superior to the so-called Fink anodes.The type of solution used, current densities employed, and other detailsof operation are entirely illustrative, as the anodes of the presentinvention are adapted for use under any conditions found suitable forthe efilcient electrowinning of manganese.

It must not be assumed that the anodes of our invention can be producedand used only in the form of cast shapes. The anodes may be extruded,rolled, drawn, extended. in short,

The origin of other constituformed in any usual way, and may even beproduced in the form of clad anodes wherein only the surface islead-silver.

We have described the anodes of our invention as being cast, but theymay be fabricated in any suitable way so long as their composition issubstantially uniform. While an alloy of lead and silver, the latter inrelatively small proportions, is eminently suitable, other alloyingconstituents not deleterious to the results desired, such as tin, forexample, may be added, and, of course, the silver content may beincreased if desired. The base of the alloy, preferably lead, must be acurrent conducting material insoluble in the anolyte, with or withoutother constituents, but containing at least a relatively smallproportion of silver to minimize the formation of insoluble manganesecompounds.

What we claim as new and desire to protect by Letters Patent of theUnited States is:

1. In the method of electrowinning manganese wherein an electrolyte ofmanganese and ammonium sulphate is electrolyzed in a diaphragm cellwherein the current efiiciency is greater than 50%, the improvementwhich is characterized by employing an anode of lead alloyed with one tothree percent silver and of less surface area than the cathode;initially operating the cell at an anode current density above about 28to about 58 amperes per square foot, and exceeding the cathode currentdensity due to the difference in their areas while maintaining thecurrent eniciency above 50%; initially depositing a protective film onthe anode under said operating conditions for a period of about 24 hoursand continuing said electrolysis under said operating conditions for aperiod of about two Weeks to form a scale composed principally of oxidesof manganese and lead at the anode and reduce the manganese deposited asoxide on the anode to less than 1% of the metal deposited on thecathode.

2. The method as set forth in claim 1 wherein the anode current densityis maintained at about 58 amperes per square foot.

HERBERT R. HANLEY. JAMES H. JACOBS.

REFERENCES CITED UNITED STATES PATENTS Number Name Date 1,759,493Tainton May 20, 1930 2,361,143 Lente et al. Oct. 24, 1944 OTHERREFERENCES Transactions of the Electrochemical Society,

